1. Field of the Invention
This invention relates generally to communication systems. More particularly, this invention relates to estimation of channel responses as involved in Orthogonal Frequency Division Multiplexing (OFDM) communication systems.
2. Description of Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme resistant to multipath interference and frequency selective fading in communication systems such as wireless local area network (WLAN) and digital audio/video broadcasting. The technique divides the channel bandwidth into multiple narrow band sub-channels or sub-carriers, which are used for transmitting data in parallel with high efficiency spectrum usage, as described in “OFDM for Wireless Multimedia Communications”, Van Nee and Prasad, Artech House Publishers, 2000.
In 1999, OFDM was selected by the IEEE 802.11 standardization committee as the technology for a 5 GHz WLAN standard—WLAN Medium Access Control (MAC) and Physical Layer (PHY) Specification: High-speed Physical Layer in the 5 GHz Band, as described in the “Information Technology—Telecommunications and Information Exchange Between Systems—Local and metropolitan area networks—specific requirements—Part 11: “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, sponsored by the LAN MAN Standards Committee of the IEEE Computer Society, ANSI/IEEE Std 802.11(a), September 1999. The IEEE 802.11(a) standard divides the 5150 MHz to 5350 MHz frequency band and the 5725 MHz to 5825 MHz frequency band into 12 20-MHz communication channels. Each of these 20-MHz channels is composed of 52 narrow band sub-carriers, which are modulated using binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16-QAM), or 64-QAM to support data rates up to 54 Mbps in 16.6 MHz occupied bandwidth on 20 MHz channelization. A typical block diagram of the baseband processing of an OFDM transceiver in the IEEE 802.11(a) WLAN is shown in FIG. 1. In the transmitter path 5, binary input data 10 are encoded by an industry standard ½ rate convolutional encoder 15. The coding rate can be increased to ⅔ or ¾ by puncturing the coded output bits to accommodate the desired data rate which is one of 6 Mbps, 9 Mbps, 12 Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps or 54 Mbps. After interleaving with an interleaver circuit 20, the binary values are mapped with a mapping circuit 25 onto BPSK, QPSK, 16-QAM or 64-QAM constellations according to the chosen data rate. A time-domain OFDM symbol is thus obtained from a Fast Fourier Transform circuit 35 by applying a length N(N=64) inverse fast Fourier transform (IFFT) to those modulation values 30, YT(0), YT(1), . . . , YT(N−1). The N complex values output from the IFFT 35 are then applied to the sub-carrier multiplexer 40 to form the baseband discrete time samples. The resulting digital samples in the time domain are then converted by the Digital-to-Analog (D/A) converter 45 to analog signals, which are further up-converted to the 5 GHz band, amplified and transmitted to the over-the-air channel 50 through an antenna.
The transmitted signal suffers from some distortions in the channel 50, which usually can be represented by a channel impulse response, h(t) 55, and an additive noise 60, ν(t) 65, in the time-domain. The resulting corrupted signal from the channel 50 is the input to the OFDM receiver 70.
The OFDM receiver 70 basically performs the reverse operations of the transmitter 5. The received RF signal is down-converted to a baseband signal and sent to an Analog-to-Digital (A/D) converter 75. The digital baseband samples are then demultiplexed by the demultiplexing circuit 80 to reconstruct the transmitted time-domain OFDM symbol structure. The digital samples of the demultiplexed signal are applied to a Fast Fourier Transform circuit 85, which creates a frequency-domain OFDM symbol with N complex values, YR(0), YR(1), . . . , YR(N−1) 90. The N complex values, YR(0), YR(1), . . . , YR(N−1) 90 are applied to a the Modulation De-Mapping circuit 95, which converts the complex values to a binary sequence. The binary sequence is the input to the De-Interleaver circuit 100, which correctly orders the sequence to reflect the original structure of the encoded transmitted data. The de-interleaved data sequence is applied to the Viterbi Decoder 105 for recovery of the transmitted data to form the receiver's output data 110.
To properly receive a data frame in burst-mode transmission, the receiver 70 first has to detect the arrival of a frame, find the unknown beginning sample instant of the first and subsequent OFDM data symbols, and, estimate/correct for any carrier frequency offset imparted to the sub-carriers due to variation in the nominal values of the oscillator frequencies in both the transmitter 5 (remote device) and receiver 70 (local device), using structured training symbols 117, 118, and 119 contained in a frame preamble 115 as shown in FIG. 2. After that, the receiver 70 can perform an FFT to convert a time-domain OFDM data symbol into N frequency-domain complex values, YR(0), YR(1), . . . , YR(N−1) 90. Ideally, these values are expected to be same as YT(0), YT(1), . . . , YT(N−1) 30, but they are usually distorted by the channel. Thus, before they are used as the input to the de-mapping 95, the values, YR(0), YR(1), . . . , YR(N−1) 90, should be compensated first so that the equalized values, ŶR(0), ŶR(1), . . . , ŶR(N−1), are the good estimations of YT(0), YT(1), . . . , YT(N−1) 30.
When a wireless channel is typically modelled in the time domain as the joint effort of multipath effects with a channel impulse response, h(t) 55 and an additive noise, ν(t) 65, the frequency-domain relation between the YT(k) and YR(k) can be expressed asYR(k)=H(k)·YT(k)+V(k), k=0 to N−1  (1)                where:                    V(k) is the frequency-domain additive noise.            H(k) is the channel transfer function which is usually called as channel state information.The channel estimation thus is simply to find the estimation of channel state information, Ĥ(k), so that ŶR(k)=YR(k)/Ĥ(k) approximates YT(k) as closely as possible.                        
Different algorithms have been proposed for channel estimations. The Minimum Mean Square Error (MMSE) based algorithms give low mean square estimation errors, but may be either impractical or not robust because they are of high computational complexity and usually require an assumption of the channel statistics. “OFDM channel estimation by singular value decomposition”, Edfors et al., IEEE Trans. On Communications, vol. 46, no. 7, pp.931-939, July 1998 describes such a technique for channel estimation. On the other hand, the Least Square (LS) estimation, which is given byĤ(k)=YR(k)/YT(k), k=0 to N−1  (2)is much simpler and easier to be implemented. However, when compared with the MMSE estimation, the LS estimation yields a higher mean square estimation error, which will translate to degradation in system performance in terms of higher bit error rate (BER) and packet error rate (PER) and should be further reduced to an acceptable level in practice.
U.S. Patent Application Publication 2001/0036235 A1 (Kadous), teaches a method and apparatus for improving LS channel estimate in OFDM communication systems. The method and apparatus allows a channel estimate to be determined independent of having knowledge on channel statistics. Channel estimation is performed by determining and then utilizing an LS estimate and an interpolation coefficient for each transmitting antenna. The interpolation coefficient is determined independently from the statistics of the channel, i.e., without needing the channel multipath power profile (CMPP). The interpolator coefficient is multiplied by an LS estimate for each transmitting antenna to determine the channel estimate for each channel.
U.S. Pat. No. 6,487,253 (Jones, IV, et al.) illustrates another method for achieving improved channel response estimation in an OFDM system in the presence of interference. The interference and/or noise present on the received training symbols are estimated first. Based on the measured noise and/or interference, a weighting among training symbols is developed. Channel response is then estimated based on a weighted least square procedure.
When these two techniques can be employed to enhance the LS channel estimation in an OFDM system, the derivation of robust interpolation coefficients or weighting factors may require multiple transmitter or receiver antennas.
In a wireless LAN system, which performs burst-mode transmission of data frames with limited length, the channel state information can be assumed to be constant within the transmission duration of a frame, and, therefore, the channel estimation is commonly performed by the use of one or more known OFDM symbols, which are usually called training symbols. In the IEEE 802.11a standard, the two long training symbols 118 and 119 contained in the preamble 115 of each data frame can be used for estimating the channel state information. It is easy to see that, by averaging the LS estimations of channel state information obtained from the prescribed training symbols, the effects of the channel noise can be reduced to some extent. By averaging the two identical long training symbols in the IEEE 802.11a WLAN, for example, the reference amplitudes and phases for doing coherent demodulation can be obtained with a noise level that is 3 dB lower than the noise level of data symbols. “An integrated 802.11(a) baseband and MAC processor”, Thomson et al., Proc. IEEE Int. Solid State Circuits Conf. (ISSCC), February 2002, pp.126-127 describes an 0.25 μm CMOS baseband and MAC processor for the IEEE 802.11(a) WLAN standard that averages the two long training symbols 118 and 119 of FIG. 2 for channel estimation/correction. U.S. Pat. No. 5,903,610 (Skold, et al.) illustrates a receiver of a digital radio communication system including a combined channel estimate that is formed by averaging a long channel estimate with a short channel estimate. Also, “Effect of channel estimation error in OFDM-based WLAN”, Cheon et al., IEEE Communication Letters, vol.6, pp.190-192, May 2002 examines the performance degradation due to the channel estimation error in an OFDM-based WLAN. The average effective Signal to Noise ratio and average bit error probabilities (BEPs) are derived in a Rayleigh fading channel. Cheon et al. have demonstrated the advantages of averaging the two long training symbols 118 and 119 of FIG. 2 for channel estimation over using a single training symbol.
The choice of the number of training symbols that can be used for channel estimation is a trade-off between a good channel estimation performance and a short training time, because the training symbols contain no actual data information. In the training structure (preamble) 115 of FIG. 2, the total training length is 16 μs. The periods t1 to t10 contain short training symbols 117 and T1 and T2 denote long training symbols 118 and 119. The dashed boundaries in the figure denote repetitions due to the periodicity of the inverse Fourier transform. A short OFDM training symbol 117 consists of 12 sub-carriers, which are modulated by the elements of a fixed spectral sequence as defined in paragraph 17.3.3 of IEEE 802.11(a) standard. A long OFDM training symbol 118 and 119 consists of 53 sub-carriers (including a zero value at dc), which are modulated by the elements of the spectral sequence as defined in paragraph 17.3.3 of IEEE 802.11(a) standard.
The long OFDM training symbols are followed by the OFDM SIGNAL symbol 120, which contains the RATE 121 and the LENGTH 123 fields of the TXVECTOR as defined in paragraph 17.2.2 of IEEE 802.11(a) standard. The encoding of the SIGNAL field 126 into an OFDM symbol 120 proceeds with convolutional encoding (R=½), interleaving, less noise sensitive BPSK mapping, pilot insertion, and OFDM modulation. The contents of the SIGNAL field 126 are not scrambled. The SIGNAL field 126 is composed of 24 bits, as illustrated in FIG. 2. The four bits 0 to 3 121 shall encode the RATE. Bit 4 122 is reserved for future use. Bits 5-16 shall encode the LENGTH field 123 of the TXVECTOR, with the least significant bit (LSB) being transmitted first. Bit 17 124 is a positive parity (even parity) bit for bits 0-16. The bits 18-23 constitute the SIGNAL TAIL field 125, and all 6 bits are set to zero. The RATE field 121 conveys information about the type of modulation and the coding rate as used for the rest of the frame. The 4 bits of RATE field 121 are set according to the values in Table 1. The LENGTH field 123 is an unsigned 12-bit integer that indicates the number of octets in the protocol service data unit that the MAC is currently requesting the PHY to transmit. This value is used by the PHY to determine the number of octet transfers that will occur between the MAC and the PHY after receiving a request to start transmission. This value is also used by the receiver 70 to determine the number of data octets contained in a received data frame.
TABLE 1RATEData Rate(4 bits)(Mbps)Modulation TypeCoding Rate11016BPSK½11119BPSK¾010112QPSK½011118QPSK¾10012416-QAM½10113616-QAM¾00014864-QAM⅔00115464-QAM¾
The OFDM SIGNAL symbol 120 is followed by variable number of OFDM DATA symbols 127 which contain actual data information. It can be seen that the long training time will reduce the efficiency of spectrum usage. It is thus useful to have a technique for achieving enhanced training capability with no extra training time required.